Is the screenshot dialog part of a python script?
Set default settings in the shortcut.

As a workaround to editing scripts, set the properties in the shortcut. As long as the operator is invoked using the shortcut, the new settings will be used. 

For screenshot the default shortcut is CtrlF3

Edit settings from  User Preferences &gt; Input &gt; Screen &gt; Save Screenshot  Uncheck "Full Screen" setting on the keymap editor.



A "helper" operator

Alternatively, can write a simple helper operator with the settings you want already set.  For example having to type in the filename each time. Sample script below saves screenshot into a folder of your choosing and simply timestamps the filename.  Change screen.screenshot to screen.area_screeny in keyset above and not have to bother with invoking the file select dialog.  The script is based on scripting &gt; templates &gt; python &gt; operator simple

import bpy

def main(context):
    from os import path
    from datetime import datetime
    directory = "/home/batfinger/Pictures/"
    filename = "Blender%s.png" % str(datetime.now())
    filepath = path.join(directory, filename)
    bpy.ops.screen.screenshot(filepath=filepath, full=False)

class SimpleOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "screen.area_screeny"
    bl_label = "Area Screeny"

    @classmethod
    def poll(cls, context):
        return True

    def execute(self, context):
        main(context)
        return {'FINISHED'}

def register():
    bpy.utils.register_class(SimpleOperator)

def unregister():
    bpy.utils.unregister_class(SimpleOperator)

if __name__ == "__main__":
    register()

    # test call
    bpy.ops.screen.area_screeny()


Note: edit the directory above to suit.